Effect of Wet−dry Cycles and Water-to-cement Ratios on Cement Paste Carbonation

Cement production is associated with significant energy consumption and CO2 emissions, whereas concrete waste from demolition and renovation has the potential for CO2 absorption. This study investigated the effects of relative humidity and water-to-cement (w/c) ratio on the carbonation of hardened cement pastes with w/c ratios of 0.4, 0.5, and 0.6. Thermogravimetric analysis, X-ray diffraction, and Fourier transform infrared spectroscopy were used to analyze the carbonated samples, which were exposed to five different constant humidity conditions and underwent five types of wet–dry cycles. The results indicated that higher w/c ratios increased the hydration degree and pore formation, facilitating CO2 diffusion and promoting carbonation. Wet–dry cycles enhanced pore generation and calcium silicate hydrates (C-S-H) decomposition through shrinkage and deformation during the drying process. In addition, the minimum and maximum humidity of the wet–dry cycles influenced the formation of vaterite and amount of vaterite converted to calcite. The highest CO2 uptake after 14 days for WC06-40100 with a w/c ratio of 0.6 exhibited twice that of RH40 and 22% higher than that of RH80 on day 14. Moreover, the amount of CO2 uptake under RH40-100 for 28 days was approximately 17% of the annual CO2 emissions from cement production.